It depends of the genuine purpose of the giving.
e.g. it sounds like you just want to wow them so you can make more sales to exising customers...this sounds self centered, which it is, but is totally normal, acceptable, common and appropriate. If thats the outcome you want then i dont think it matters what you send them, your just trying to get to the front of their minds again. Might as well go for the cheap and easy option e.g. a hand written thank you card.

On the other hand, if you have some customers that you genuinely care about (for more than just their future patronage) then a more elaborate gift may be appropriate.